This invention relates to slip casting, and is more particularly concerned with a new clay composition for use in slip casting, a slip casting composition incorporating the same and a slip casting method using such a slip casting composition.
The process of slip casting is widely used in the traditional ceramic industries of sanitary ware and tableware, to fabricate items of complex shape that are difficult to make by other methods.
In the slip casting process, a slurry or slip is prepared from a mixture of suitable raw materials, usually china clay (a kaolinitic clay), ball clay, quartz and feldspar. The slurry is dispersed at a solids content typically in the range 70-75 wt % (approximately equivalent to slip densities of 1750-1850 g.dm.sup.-3), and deflocculants (dispersants) are added during dispersion to control the rheological and casting properties of the slip.
The slip is then poured into a mold which is normally made of plaster of paris, and allowed to remain in the mold for a period of time depending on the thickness of cast required, the temperature and rheological properties of the slip and the nature of the raw materials. For tableware, where pieces need only be 2-3 mm thick, the casting time may be between 5 and 15 minutes. For sanitary ware, where cast thickness of about 9 mm are required, the casting time may be between 45 and 90 minutes.
During the casting period, water is sucked out of the slip by the capillary action of the mold, and a skin or cast of clay body is formed on the inner surface of the mold. After the required thickness of cast has been built up, the remaining slip is drained off and the cast is allowed to dry for a period of time before the mold is opened and the case removed. For tableware this drying period may be less than one hour, but for sanitary ware it can be several hours.
The manufacture of sanitary ware by slip casting is a relatively slow process. In many factories, only one cast per day is obtained from each mold, and even in factories that have automated systems for speeding up cast and mold drying, rarely are more than two casts per mold per day achieved. This means that many molds have to be used in order to reach an adequate level of production, and the process is quite labour intensive.
It has always been a long term objective of the sanitary ware industry to speed up the casting process, and hence improve productivity and reduce costs. There are several ways to increase the casting rate of a slip.
The rheology of the slip can be changed to increase the slip thixotropy ie. the time dependent increase in viscosity. This results in an increase in cast permeability and moisture content. However, increasing the thixotropy of the slip can also result in problems with slip drainage at the end of the casting period, and can give soft flabby casts that distort or collapse when removed from the mold, or which crack during drying.
Increasing the temperature of the slip will increase the casting rate, but too high a slip temperature may result in excessively fast drying and cracking of the cast pieces.
Increasing the slip density may also increase the casting rate under certain conditions, but this is not infallible. Increasing the slip density usually results in a decrease in cast porosity and hence permeability, counteracting any potential increase in casting rate. The slip rheology will also change, leading to problems with drainage after casting, and general slip handling in the factory.
Applying a pressure to the slip increases the casting rate in proportion to the applied pressure. `Pressure casting` as it is known, is a fairly recent manufacturing process that is becoming more popular, although the capital cost of the equipment is high. The method is commonly limited to simple two piece molds such as wash basins, and traditional plaster casting has still to be used for many items, notably closets.
The most common method adopted for increasing casting rate is by changing the types or proportions of clay in the body to give a more permeable cast. The total amount of clay in the recipe can be reduced, or the amount of ball clay can be reduced relative to the amount of kaolin or china clay, or coarser china clays or ball clays can be substituted. These types of recipe alteration invariably result in a decrease in cast strength and cast plasticity, leading to an increase in cracks during trimming and drying, and breakages when handling. Thus any increase in overall output due to faster casting may be offset to some degree by an increase in losses.
Whatever the method of formation, all cast pieces are subject to the same processing after casting. This means that they have to withstand the same trimming, handling and glazing processes, and so have to have the same plasticity and strength before firing. Unfired vitreous sanitary ware has a strength approximately in the range 1.25-1.65 MPa when conditioned in an atmosphere of 80% relative humidity, or 2.5-3.3 MPa when dried at 110.degree. C. It is therefore important that a minimum conditioned strength of 1.25 MPa be maintained, independent of any changes in casting rate that might be achieved by alteration of the clays or casting process.